Optical waveguide for second harmonic generation

Abstract: An optical waveguide device for use as a second harmonic generator includes a nonlinear optical crystalline substrate of KTiOPO₄ and an optical waveguide core of a dielectric material, such as Ta₂O₅, disposed on the nonlinear optical crystalline substrate. The optical waveguide converts a fundamental wave into a second harmonic wave, the second harmonic wave having the same polarization as the fundamental wave. The optical waveguide has a thickness of 190 nm or greater and a refractive index difference greater than 0.1. The dispersion of the guided modes is used for phase matching. The optical waveguide device also has a dielectric loaded strip disposed on the optical waveguide core. The dielectric loaded strip may be made of SiO₂ or SiO₂ doped with Ta₂O₅. Alternatively, the optical waveguide may be a ridge-type optical waveguide, and the optical waveguide device may also has a cladding layer disposed on the nonlinear optical crystalline substrate and the optical waveguide core.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to an optical waveguide device for use as a wavelength converter or a second harmonic generator for converting fundamental light into a second harmonic wave having the wavelength which is half that of the fundamental wave, and more particularly to an optical waveguide device for use as a light source for applying a short-wavelength coherent light beam to an optical disc to record information thereon at a high recording density. Description of the Related Art: Optical waves that can be guided in the optical waveguide are limited to an intrinsic mode having discrete propagation constants and can be distinguished by modal orders 0, 1, 2, ... In general, nth-order intrinsic mode has n nodes in its electric field distribution and an intensity distribution of a far field pattern of light emitted from the waveguide has n nodes accordingly. The existence of nodes is very disadvantageous particularly for devices which focus the emitted light. Therefore, it is customary that optical waveguide devices are designed to utilize a 0th-order waveguide light in which no nodes exist. Wavelength converters using an optical waveguide, for example, are known as one of such optical waveguide devices. In this case, the above-mentioned characteristic imposes a large restriction on designing the devices. There have been known optical waveguide devices for use as wavelength converters or second harmonic generators for converting fundamental wave having a wavelength λ into a second harmonic wave having a wavelength λ/2. The conventional optical waveguide devices are typically classified into three types depending on how the phase matching condition is achieved to generate the second harmonic wave efficiently. The optical waveguide devices in the first category are known as optical waveguide devices of the guided mode type. In the optical waveguide devices of the guided mode type, phase matching is accomplished by bringing the effective refractive index of a fundamental guided mode and the effective refractive index of a second harmonic guided mode into agreement with each other by using modal dispersions. Nonlinear optical materials which satisfy such a condition are limited, and there have been known no optical materials that would practically achieve a high conversion efficiency in the optical waveguide devices of this group. Also, even when the phase-matching condition is satisfied, a resultant second harmonic wave becomes a high-order waveguide mode of first-order or greater in most cases and is not suitable for being focused in use. The second class of optical waveguide devices is referred to as quasi-phase matching (QPM) optical waveguide devices which achieve quasi-phase matching by periodically inverting the spontaneous polarization of a nonlinear optical material. The QPM optical waveguide devices are advantageous in that there is available a wide range of optical materials to choose from relatively freely, and a high conversion efficiency can be expected and the 0th-order mode can be used. However, since they require a minute periodic inverted structure to be formed in themselves, a strict control process is needed in the fabrication of the QPM optical waveguide devices. It is difficult to manufacture the QPM optical waveguide devices with good reproducibility with the present fabrication technology. The optical waveguide devices which belong to the third kind are called Cerenkov-radiation optical waveguide devices (see e.g. EP-A-0 452 828) that convert a fundamental guided mode into a second harmonic radiation mode. The Cerenkov-radiation optical waveguide devices are advantageous in that the phase matching condition therefor is much looser than those for the optical waveguide devices of the first and second types, and they are simple in structure. However, their conversion efficiency is comparatively low, and they fail to produce a high second harmonic optical output power. Another problem of the Cerenkov-radiation optical waveguide devices is their poor focus characteristics because of a specially shaped pattern of radiation. As described above, there is available only a limited range of optical materials for the optical waveguide devices which achieve phase matching of the guided mode type. Presently reported examples of such materials are as follows: "Applied Physics Letters", vol. 24, p. 222 - (1974), shows an optical waveguide device comprising a linear thin-film waveguide of TiO 2 formed on a nonlinear optical crystalline substrate of quartz. The optical waveguide device converts a fundamental wave having a wavelength of 1.06 µm in a TE 0 (0th order) into a second harmonic wave in a TM 0 mode (0th order). Since the nonlinear optical coefficient of the substrate is very small, the second harmonic wave has an output power of about 10 mW when the fundamental wave has an input power of 100 W. Therefore, the optical waveguide device has a conversion efficiency of only about 10 -6 % and is not particularly usable. This article does not report the generation of a second harmonic wave having a wavelength in the blue region from a fundamental wave having a wavelength of 1.0 µm or shorter. "Optics Communications", vol. 15, p. 104 ∼ (1975), discloses another optical waveguide device comprising a nonlinear thin-film waveguide of ZnS formed on a nonlinear substrate of LiNbO 3 . The optical waveguide device converts a fundamental wave having a wavelength of 1.1 µm in a TE 0 mode into a second harmonic wave in a TE 2 (2nd order). However, the conversion efficiency is very low as the second harmonic wave has an output power of about 5 µW when the fundamental wave has an input power of 70 W. No blue light is produced as the second harmonic wave. Also, the resultant second harmonic wave is in the second-order mode. "Journal of Crystal Growth", vol. 45, p. 355 ∼ (1978), discloses still another optical waveguide device comprising a nonlinear thin-film waveguide of LiNbO 3 formed on a linear substrate of MgO. The optical waveguide device converts fundamental waves having respective wavelengths of 1.06 µm and 0.86 µm in a TM 0 mode into second harmonic waves in a TM 1 mode (1st order) and a TM 2 mode (2nd order). However, the conversion efficiency is also very low as the second harmonic wave has an output power of about 1 µW when the fundamental wave has an input power of 10 mW, and only a second harmonic wave of high-order can be obtained. In the EP-A-0 433 487 a second harmonic wave generating device is disclosed, wherein a fundamental wavelength, a thickness of a thin film waveguide layer, an ordinary refractive index of the thin film waveguide layer at the fundamental wavelength, an extraordinary refractive index of the substrate at a second harmonic wavelength, and an extraordinary refractive index of the thin film waveguide layer at the second harmonic wavelength are represented by a certain equation. In one preferred embodiment the width of the thin film waveguide layer is smaller than the wavelength of the second harmonic wave in air. The reported optical waveguide devices which generate a second harmonic wave of the same polarization by phase matching of the guided mode type have an impractical low conversion efficiency. Their conversion efficiency at the time they convert a fundamental wave into a second harmonic wave in a short wavelength range, such as blue light, is extremely low. Further, the second harmonic wave becomes a high-order mode and nodes are produced in the intensity distribution of emitted light. Therefore, the above optical waveguide device is not suitable for use as the wavelength converter. For the above reasons, almost no optical waveguide devices of the guided mode type have subsequently been reported so far. Rather, research efforts have been directed to optical waveguide devices which achieve phase matching of the QPM type that can generate a second harmonic wave of a 0th-order mode because of greater freedom for the selection of available optical materials. A present theme assigned to the QPM type is to manufacture devices with excellent reproducibility. As described above, in the conventional optical waveguide device, since a guided light of high-order mode has nodes corresponding to orders in the intensity distribution of the emitted light, such nodes are large restriction in designing a wavelength converter which is one of the examples to which the optical waveguide devices are applied. OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a second-harmonic generation optical device in which no nodes exist in an intensity distribution of emitted light of guided light of high-order mode. According to the present invention, there is provided a second-harmonic generation optical device as defined in claim 1. The nonlinear optical crystalline substrate may be made of KTiOPO 4 . The dielectric material may comprise Ta 2 O 5 . The optical waveguide device may further be comprised of a dielectric loaded strip disposed on the optical waveguide core. The dielectric loaded strip may be made of SiO 2 or SiO 2 doped with Ta 2 O 5 . Alternatively, the optical waveguide may be comprised of a ridge-type optical waveguide, and the optical waveguide device may further be comprised of a cladding layer disposed on the nonlinear optical crystalline substrate and the optical waveguide core. The cladding layer may be made of SiO 2 or SiO 2 doped with Ta 2 O 5 . The optical waveguide may have a thickness of at least 190 nm. According to the present invention, even when a guided-wave mode is the high-order mode of first-order or greater, the guided light is confined in a range smaller than the wavelength thereof in the air in the optical waveguide core and then guided in the optical waveguide device. Thus, when the guided light is emitted to the outside, it has an intensity distribution of a single peak response having no nodes. Further, according to the present invention, if the substrate is made of the nonlinear optical crystalline substrate and is applied to the wavelength converter based on this optical waveguide device, then a portion of the fundamental wave having the wavelength λ that was introduced into the optical waveguide is converted into the second harmonic wave having the wavelength λ/2 in the optical waveguide core, and this second harmonic wave is guided in the optical waveguide core and emitted to the outside. In this case, even when the generated second harmonic wave is in the high-order mode having nodes, such second harmonic wave is confined in a region smaller than the wavelength thereof in the air in the optical waveguide core and guided. Thus, when the second harmonic wave is emitted to the outside, it has an intensity distribution of single peak response having no nodes. Therefore, the optical waveguide device according to this invention can be suitable for use as the wavelength converter. When a fundamental wave having a wavelength λ is introduced into the optical waveguide and guided therethrough, a portion of the fundamental wave is converted into a second harmonic wave having a wavelength λ/2, which is emitted out of the optical waveguide. Phase matching which is achieved for such optical conversion is of the guided mode type wherein the effective refractive index of a fundamental guided mode and the effective refractive index of a second harmonic guided mode are brought into agreement with each other by modal dispersions. If the nonlinear optical crystalline substrate is made of KTP, in case of the fundamental wave and the second harmonic wave have the same polarization, the optical waveguide device according to the present invention can convert an inputted fundamental TE mode into a second harmonic TE mode, and also can convert an inputted fundamental TM mode into a second harmonic TM mode. Therefore, the largest nonlinear optical coefficient d 33 among the nonlinear optical coefficients of KTP can contribute to increasing a wavelength conversion efficiency, with the result that the conversion efficiency with which the fundamental wave is converted into the second harmonic wave is greatly increased. When the nonlinear optical crystalline substrate was made of KTP and the dielectric material of the optical waveguide core was Ta 2 O 5 , the optical waveguide device according to the present invention was able to convert a fundamental wave having a wavelength of 830 nm and a power of 71 mW into a second harmonic wave having a wavelength of 415 nm and a power of 5.4 mW and polarized in the same direction as the fundamental wave. The normalized conversion efficiency of the optical waveguide device was 640 %/Wcm 2 . As described above, according to the present invention, the optical waveguide device can generate light having an intensity distribution of single peak response having no nodes even when the guided light is in the high-order mode. Therefore, when the optical waveguide device according to the present invention is applied to the wavelength converter, the optical waveguide device can generate a coherent light beam having a short wavelength which has a single peak intensity distribution. Therefore, the optical waveguide device according to the present invention can generate a coherent light beam having a relatively short wavelength, and can be used as light source for applying such a coherent light beam to an optical disc to record information thereon at a high recording density. The above and other objects, features, and advantages of the present invention will become apparent from the following description of illustrative embodiments thereof to be read in conjunction with the accompanying drawings, in which like reference numerals represent the same or similar objects. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged schematic perspective view of an optical waveguide device according to an embodiment of the present invention; FIG. 2 is a modal dispersion graph according to the present invention; FIG. 3 is an electric field distribution diagram according to the present invention; FIG. 4 is a diagram showing a far field pattern emitted to the air out of an exit end of the optical waveguide device according to the present invention; FIGS. 5A through 5E are enlarged schematic perspective views showing a process of manufacturing the optical waveguide device shown in FIG. 1; FIGS. 6 and 7 are perspective views showing optical waveguide devices according to other embodiments of the present invention, respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. 1, an optical waveguide device according to an embodiment of the present invention is comprised of a nonlinear optical crystalline substrate 1 in the form of a single-crystal a plate of KTP, i.e., a substrate having a planar surface lying perpendicularly to the a axis of a crystal, an optical waveguide core 12 in the form of a thin film of Ta 2 O 5 deposited on the nonlinear optical crystalline substrate 11, and a dielectric loaded strip 13 disposed on the optical waveguide core 12, the dielectric loaded strip 13 being made of SiO 2 or SiO 2 doped with Ta 2 O 5 . The optical waveguide device is thus of the strip loaded optical waveguide type. FIG. 2 shows numerical data of modal dispersion curves obtained when a line width W of the pattern of the optical waveguide core 12 was selected to be 3.2 µm and the wavelength λ of the fundamental wave was selected to be 830 nm. In FIG. 2, a curve representing the fundamental TE 0 mode and a curve representing the second harmonic mode cross each other at a point where the Ta 2 O 5 film thickness is 225 nm. More specifically, phase-matching of the guided mode type is achieved. In such optical waveguide, the wavelength of the fundamental wave in the TE 0 mode is converted into that of the second harmonic wave in the TE 1 mode. In this case, the second harmonic TE 1 mode has one node, as shown by an electric field distribution in FIG. 3, because the first-order mode exists in the Ta 2 O 5 film thickness direction. However, a refractive index of Ta 2 O 5 forming the optical waveguide core 12 is 2.309 which is larger than a refractive index 1.947 of KTiOPO 4 by 0.362 and which is also larger than a refractive index 1.47 of SiO 2 forming the dielectric loaded strip, i.e., the cladding layer 13 by 0.839. Also, the Ta 2 O 5 film thickness of the optical waveguide core 12 is 225 nm which is sufficiently much smaller than the wavelength 415 nm of the second harmonic wave in the air. Therefore, the guided light is confined in a region sufficiently smaller than the wavelength of the second harmonic wave in the air and then guided. Thus, when the guided wave is emitted to the air out of the exit end of the optical waveguide device, the far field pattern thereof has a single peak intensity distribution as shown in FIG. 4. Further, when a length L of the optical waveguide device was selected to be 4.1 mm, a second harmonic wave of a power of 9.5 mW was obtained from a fundamental wave of a power of 96 mW. This is a normalized conversion efficiency of 610 %/Wcm 2 and is one of the highest normalized conversion efficiencies among presently recognized conversion efficiencies of the wavelength converter of optical waveguide device type. A process of manufacturing the optical waveguide device shown in FIG. 1 will be described below with reference to FIGS. 5A through 5E. First, as shown in FIG. 5A, a thin film of Ta 2 O 5 is deposited as an optical waveguide core 12 on a nonlinear optical crystalline substrate 11 in the form of a single-crystal a plate of KTP which has been optically polished, by low pressure chemical vapor deposition (CVD) method, for example. The thin film of Ta 2 O 5 should preferably have a thickness T of 190 nm or greater. Then, as shown in FIG. 5B, a dielectric layer 13 made of SiO 2 or SiO 2 doped with Ta 2 O 5 is deposited to a thickness of 500.0 nm, for example, on the thin film of Ta 2 O 5 by sputtering or the like. As shown in FIG. 5C, a mask 4 of a photoresist or the like which is of a predetermined linear pattern is thereafter formed on the dielectric layer 13. Specifically, for example, a photoresist is coated on the entire surface of the dielectric layer 13, and then exposed and developed into the linear mask 4. The mask 4 should preferably have a width W ranging from 1 to 5 µm. Then, the dielectric layer 13 is wet-etched with an etchant of buffered hydrofluoric acid thereby forming the dielectric loaded strip 3 that is patterned into a linear configuration having the width W. If the etchant is comprised of a mixture of hydrofluoric acid and ammon fluoride at a volume ratio of 12 : 100 and is used at room temperature, then the dielectric layer 13 can highly selectively be wet-etched in about 3 minutes because the etching rate of SiO 2 is about 170 nm/min. and the etching rate of Ta 2 O 5 is 0.1 nm/min. or less. The etching rate of SiO 2 doped with 40 mol% of Ta 2 O 5 is 1.3 nm/min. Therefore, the dielectric layer 13 of SiO 2 doped with 40 mol% of Ta 2 O 5 can also selectively be wet-etched. Thereafter, as shown in FIG. 5E, the mask 4 is removed, thus completing the strip loaded optical waveguide device. When a fundamental wave having a wavelength λ is introduced into the waveguide core layer 12, the fundamental wave is confined in a region of the waveguide core layer 12 where the dielectric loaded strip 13 is formed, i.e., the optical waveguide. The fundamental wave is guided through the optical waveguide, and a portion thereof is converted into a second harmonic wave having a wavelength λ/2, and emitted from the exit end of the optical waveguide. In an example, the width W of the dielectric loaded strip 13 was 3.2 µm, the thickness T of the optical waveguide in the form of a thin film of Ta 2 O 5 was 225.0 nm, and the optical waveguide had a length L of 4.1 mm. When a fundamental TE guided mode having a wavelength of 830 nm and a power of 96 mW was guided in such an optical waveguide device, the optical waveguide device generated a second harmonic TE guided mode having a wavelength of 415 nm and a power of 9.5 mW. The normalized conversion efficiency of the optical waveguide device was 610 %/Wcm 2 , which is one of the highest conversion efficiencies among the presently recognized conversion efficiencies of the optical waveguide devices. The optical waveguide device according to the present invention is therefore capable of achieving phase matching of the guided mode type as well as phase matching of the Cerenkov-radiation type for a high conversion efficiency by varying the thickness and width of the optical waveguide. As described above, according to the embodiments of the present invention, even when the waveguide mode is a high-order mode of first-order or higher, the waveguide mode is confined in the region smaller that the wavelength thereof in the air in the optical waveguide core 12. Therefore, when the guided light beam is emitted to the outside, it has a single peak intensity distribution having no nodes. Further, according to the embodiments of the present invention, if the substrate 11 is made of the nonlinear optical crystalline substrate and the optical waveguide according to this embodiment is applied to the wavelength converter, then a portion of the fundamental wave having the wavelength λ that was introduced into the optical waveguide core 12 is converted in wavelength into the second harmonic wave having the wavelength λ/2 while being guided in the optical waveguide core 12. Then, this second harmonic wave is guided in the optical waveguide core 12 and emitted to the outside. In this case, even when the generated second harmonic wave is in the high-order more having nodes, the second harmonic wave is confined in the region smaller than the wavelength thereof in the optical waveguide core 12 and then guided. Therefore, when the second harmonic wave is emitted to the outside, it has a single peak intensity distribution having no nodes. Thus, the optical waveguide device according to the present invention can be suitably applied to the wavelength converter. Further, according to the embodiments of the present invention, if the substrate is made of KTiOPO 4 , then optical materials that should preferably have a refractive index larger than that of the KTiOPO 4 by 0.1 or greater to form the optical waveguide core 12 can be selected from a wider variety of optical materials because the refractive index of the KTiOPO 4 is small as compared with those of the nonlinear optical crystals. Also, if the fundamental TE mode is converted into the second harmonic TE mode or the fundamental TM mode is converted into the second harmonic TM mode, then the largest component d 33 among the nonlinear optical constants of the KTiOPO 4 , thereby increasing the conversion efficiency with which the fundamental wave is converted into the second harmonic wave extremely. Furthermore, according to the embodiments of the present invention, if the substrate is made of KTiOPO 4 and the optical waveguide core 12 is made of a dielectric material mainly made of Ta 2 O 5 , then the second harmonic wave of a power of 9.5 mW was obtained from the fundamental wave of a power of 96 mW with a single peak intensity distribution. As described above, according to the embodiments of the present invention, the optical waveguide device can generate emitted light having a single peak response intensity distribution in which no nodes exist even when light is a guided light of high-order mode. Therefore, when this optical waveguide device is applied to the wavelength converter, the wavelength converter can generate a coherent light beam having a short wavelength with a single peak response intensity distribution. FIG. 6 shows an optical waveguide device according to another embodiment of the present invention. In the embodiment shown in FIG. 6, the optical waveguide device is of a ridge-type structure, rather than of the strip loaded type, and includes an optical waveguide core 12 made of Ta 2 O 5 or the like patterned into a linear configuration on a nonlinear optical crystalline substrate 1 made of KTP or the like. Also, as shown in FIG. 7, the optical waveguide core may be a planar waveguide core and the arrangement of the optical waveguide device of the present invention may be freely modified with similar action and effects being achieved. In each of the embodiments described above, the nonlinear optical crystalline substrate and the optical waveguide core may be made of a dielectric material other than KTP a -plate and Ta 2 O 5 . The optical waveguide device according to the present invention may be used as a light source for generating and applying a short-wavelength coherent light beam to an optical disc or the like to record information thereon at a high recording density. Since the optical waveguide device according to the present invention can be manufactured much more simply than the optical waveguide devices for achieving phase matching in the QPM mode, it can be manufactured with an improved yield at an improved production rate. Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications could be effected therein by one skilled in the art without departing from the scope of the invention as defined in the appended claims.

Claims: A second-harmonic generation optical device for converting a fundamental electromagnetic wave into a second harmonic electromagnetic wave, comprising: a substrate (1) made of a first optical material, and an optical waveguide core (12) disposed on said substrate (1) and made of a second optical material, wherein one of said first or second optical material is a nonlinear optical material and the refractive index of said second optical material is greater than the refractive index of said first optical material by 0,1 or more and the thickness of said optical waveguide core (12) is smaller than the wavelength of the second harmonic wave in air, characterized in that the width of said optical waveguide core (12) is chosen such that an effective refractive index at the frequency of the fundamental wave is about equal to that at the frequency of the first or higher order mode of the second harmonic wave and such that the far field pattern of the second-harmonic output beam shows a single peaked intensity profile. A device according to claim 1, characterized in that said first optical material is KTiOPO 4 . A device according to claim 1 or 2, characterized in that the second optical material is Ta 2 O 5 . A device according to claims 1 to 3, characterized in that the wavelength of the fundamental wave is about 830 nm and the thickness of said optical waveguide core is about 225 nm. A device according to claim 1, further comprising a dielectric loaded strip (13) disposed on said optical waveguide core. A device according to claim 5, wherein said dielectric loaded strip is made of SiO 2 . A device according to claim 5, wherein said dielectric loaded strip is made of SiO 2 doped with Ta 2 O 5 . A device according to claim 1, wherein said optical waveguide comprises a ridge-type optical waveguide and said optical waveguide core is fabricated in a linear fashion. A device according to claim 8, further comprising a cladding layer disposed on said nonlinear optical crystalline substrate and said ridge-type optical waveguide core and said cladding layer is made of SiO 2 . A device according to claim 8, further comprising a cladding layer disposed on said nonlinear optical crystalline substrate and said ridge-type optical waveguide core and said cladding layer is made of SiO 2 doped with Ta 2 O 5 .

Inventors: DOUMUKI TOHRU, SAITOH MASAKI, TAMADA HITOSHI, DOUMUKI, TOHRU, SAITOH, MASAKI, TAMADA, HITOSHI, DOUMUKI, TOHRU, C/O SONY CORPORATION, SAITOH, MASAKI, C/O SONY CORPORATION, TAMADA, HITOSHI, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1994
Main classification: G02F1
IPC codes: G02F1, G02B6